Typically used on a baby boomer or someone older than the person saying this phrase, it's used to let the baby boomer/ person older than you know you're talking shit or you're being annoying
Ok Boomer,
Karen: Omg kids these days have no respect. OK BOOMER
Boomer or “baby boomer” Is often described as someone who was born in 146-1964. Now it is used as an insult to someone who is a normie and knows nothing meme wise